1. Field of the Invention
The present invention relates to drills, and is more particularly concerned with microdrills as may be used in drilling circuitboards.
2. Description of the Prior Art
When microdrilling, for example on the order of 0.004"-0.025", in the circuitboard industry, a different set of drilling criteria must be addressed and evaluated in that drill breakage becomes a major factor. There are many forces which act on a microdrill when it is in the cut. Any of these forces, or a combination thereof, may cause drill breakage if they exceed the strength of the drill. Some of the major forces affecting drill breakage are the torque from the cutting edges, the thrust from in-feed, the friction on the hole wall, deflection and the chip moving force. Rupture results when the total force on the microdrill exceeds the minimum cross-sectional rupture strength at the point of stress concentration.
The above forces are those which originate in the actual cutting process. Other forces which add to breakage are those stemming from machine mechanics, rigidity of fixture, harmonic vibration and material composition. If it were possible to eliminate all of the negative forces external to those which originate in the actual cutting process, then conventional circuitboard drill geometry would prevail. However, because microdrill cross-sectional areas are so small, their rupture strengths are proportionately low. For this reason, it is imperative that the drill be designed so that it may withstand as much of the total negative forces as possible.
It is impractical for the user to completely control all of the operational forces such as zero spindle runout, absolute perpendicularity of drill to workpiece, homogeneity of circuitboard material, etc. These parameters must be closely controlled; however, it is the drill manufacturer's responsibility to produce a microdrill that can operate under production requirements.
There are major differences in microdrilling when it is compared to standard circuitboard drilling which requires different approaches to geometry and materials in the microdrill itself.
The standard practice for surface footage is to operate between 600-700 surface feet per minute (SFM). A #57 drill (0.043") would be used at between 53,000-62,000 rpm. Since 6,00014 80,000 rpm is the limit of most production equipment presently in use, the SFM of a #97 drill (0.0059") would be 92-122 SFM or approximately 14% of a #57 drill. In order to run a #97 microdrill at 700 SFM, 453,000 rpm would be required.
It is standard practice in the industry to mount a plurality of microdrills and operate the drilling machine somewhat in the manner of a punch press with a stroke of up to, for example, 200 strokes per minute. Therefore, microdrills must be operated at fairly high rpm.
The ratio of the length of a column as compared to its diameter, or as applied to a drill, the body length to the drill diameter, is known as the aspect ratio. As an example, the aspect ratio of a #57 drill would be 9.3 to 1. On a #97 drill, although body length is shortened somewhat, the aspect ratio is 16.9 to 1.
The chisel edge cutting zone is a negative rake cutting zone that is pushing or grinding material instead of shearing. This is a cutting area of high thrust force and heat. As can be demonstrated, this zone is disproportionate in a microdrill when compared with a standard #57 drill. This relative increase in the high temperature grinding zone can cause premature wear on the cutting edges which will cause breakage due to increased torque forces.
The weakest section of the drill under stress is at the rear of the fluted portion, where the fluted web carries into the drill body. Conventional microdrill structure provides, as disclosed for example by Andreas Maier in his U.S. Pat. No. 4,080,093, the flute terminus location be on a conical tapered section which connects the shank to the drill body. On microdrills, as well as all drills, it is imperative that the web is tapered along the flute, with a larger web at the rear. This helps to increase the strength of the drill at this critical point.
Another possible cause of breakage in microdrills is the friction created between the drill body and the walls of the hole being drilled. This can be alleviated by providing backtaper along the drill body. However, backtaper must be precisely controlled within close limits to ensure that a sufficient strength is retained at the back of the drill, yet provide the benefits of backtaper.
As will be appreciated from the detailed description below, drilling problems of the type mentioned above, such as minor spindle misalignment, inconsistencies in the board material, and irregularities in the entry material surface can create bending forces on the drill at the shoulder of the drill body on a common shank drill where the flute is carried out.
On drills where the diameter is of sufficient size, such as a #57 drill, the drill has sufficient strength at this point to resist bending. However, on microdrills where the drill diameter is much smaller, these forces will deflect the drill. This creates a maximum stress at the shoulder of the drill body. If the flute carries out and is terminated at this point of maximum stress the drill will have a tendency to fail at this point.